I have perused the documentation and found that currently one can only load a uxml file via path. That is fine if one wants to deploy the elements directly from a Unity script, but problematic if one wants to deploy editor scripts through an external dll. Is there a way or a plan to allow this via stream ?
In the case of publishing DLLs without access to Unity assets the way to go is to use the C# API, which allows you to create elements the same way you can with UXML.
Another option which may deserve a look is to pre-build UXML assets into an asset bundle that you can then embed as a resource of your DLL.
This post contains relevant information since there was a similar question before: